http://win.niddk.nih.gov/publications/tools.htm
Weight-Control Information Network, National Institute of Diabetes and Digestive and Kidney Diseases
Health care providers use body mass index (BMI) and waist circumference measures to assess a person's risk of developing diabetes, heart disease, or other health problems. This fact sheet tells you how to measure your BMI and waist circumference, and what these measures mean for your health.